1) Field of the Invention
The present invention relates to a technique for recovering from data loss when storage media fail in a hierarchical storage system in which a plurality of storage devices is combined.
2) Description of the Related Art
There has been a technique that the same data is written on the pair of disks as done in a RAID (Redundant Arrays of Inexpensive Disk) system to make the pair of disks be in a mirroring relationship, whereby data on one of the disks can be restored on the basis of data on the other disk when one of the disks fails.
However, such technique has a problem that when the pair of disks in the mirroring relationship both fail, the data cannot be restored, hence the data is lost.
For the above reason, there has been proposed a technique as follows (see Japanese Patent Application Laid-Open No. 2003-316525). Namely, besides the pair of storage units (the first and second storage units) in the mirroring relationship, the third storage unit is written thereon the same data as that on the pair of storage units, as well. When a fault (failure) occurs in the first storage unit, the second and third storage units configure a disk array of RAID 1. When a fault also occurs in the second storage unit, the data is kept by the third storage unit, whereby the fault (failure) tolerance is improved.
Meanwhile, there is a hierarchical storage system, using a characteristic that data accessed by the host computer (data processing apparatus) is classified into data having high access frequency and data having low access frequency. As shown in FIG. 7, the hierarchical storage system adopts a data hierarchical structure in which the data having high access frequency is recorded on an expensive primary storage device having high performance but small capacity located closer to the host computer, whereas the data having low access frequency is recorded on an inexpensive secondary storage device having low performance but large capacity located farther from the host computer.
In such hierarchical storage system, there is used a library apparatus configured by accommodating a plurality of recording media such as magnetic tapes, optical disks or the like as the secondary storage device. This hierarchical storage system has a function called a virtual disk mechanism which allows the host computer to virtually regard a storage area having a large capacity formed by the recording media in the library apparatus as a disk space, thereby providing, to the host computer, a disk space larger than the physical disk possessed by the primary storage device.
Meanwhile, a logical disk space of all storage areas on the secondary storage device represented to the host computer by means of the virtual disk mechanism is referred to as a virtual logical volume (VLU: Virtual Logical Unit). To the contrary, a logical disk space configured by a physical disk of the primary storage device is referred to as a logical volume (OLU: Open system Logical Unit). The user can voluntarily set the capacities and the numbers of the virtual logical volumes and the logical volumes.
In the primary storage device of the hierarchical storage system having the virtual disk mechanism described above with reference to FIG. 7, a plurality of physical disks are in the mirroring relationship. When one of the plural physical disks fails, it is possible to keep data by another physical disk.
However, when the plural physical disks in the mirroring relationship concurrently fail, the data cannot be kept. The maintenance operation such as replacement of the physical disks or the like is required to use again the logical volume on the failed physical disks. Hence, it is impossible to use the logical volume, immediately.
To use again the logical volume on the failed logical disks, it is necessary to read the data from the secondary storage device onto the physical disks after the replacement of the logical disks. However, it takes a long time to perform the operation of reading the data from the secondary storage device. A reason of this is that it is necessary to perform steps of taking out a recording medium (magnetic tape, optical disk or the like) holding the data from a shelf storing a plurality of recording media by an accessor, conveying the storage medium and inserting it to a drive unit to access to the recording medium, and reading out the data from the recording medium by the drive unit, in order to read the data from the secondary storage device (library apparatus).